The present invention relates to fusion bonding of glass plates, particularly to vacuum fusion bonding of patterned glass plates which include etched features such as microstructure channels, and more particularly to an improved apparatus and method for vacuum fusion bonding of large area patterned glass plates.
Apparatus using a network of fluidic channels, such as microchannel arrays for chemical electrophoresis, involve the formation of the channels on one or both mating glass plates, for example, and the plates are bonded together, such as by fusion bonding. A common problem with fusion bonding is incomplete bonding of the areas, leaving thin, unbonded regions or voids. Even with the use of external force and pressure, voids are difficult to avoid. The concept of developing a partial vacuum between two substrates while maintaining an ambient or higher pressure outside the substrates has been used in mask aligners for some time, and this greatly enhances the area of close contact. The concept of using vacuum and fusion has been demonstrated on glass, and that process is described and claimed in copending U.S. application Ser. No. 09/067,022, filed Apr. 27, 1998, entitled "Vacuum Pull Down for an Enhanced Bonding Process", assigned to the same assignee.
Recently, an apparatus and method for vacuum fusion bonding of patterned glass plates, such as microstructure channels for chemical electrophoresis has been developed, and which eliminated intermediate bonding layers and solved the problem of conveniently applying the vacuum force with an apparatus that is compatible with high temperatures needed for glass fusion bonding and had potential use for very large substrates that are patterned with small features. This apparatus and method are described and claimed in copending U.S. application Ser. No. 09/039,522 filed Mar. 16, 1998, entitled "Vacuum Fusion Bonding of Glass Plates", assigned to the same assignee. While this apparatus and method of vacuum fusion bonding produced excellent glass to glass bonds, the apparatus has some practical limitation in ease of use, maintenance, and alignment. In that apparatus and method, the glass plate pair (top and bottom glass plates) was bonded with the plate which contained holes for pulling a vacuum in it being on top of the microchannel plate, which in turn, was placed on top of the main steel platform used for fusion bonding. At elevated bonding temperatures, the glass becomes soft enough to cause the surface texture of the steel platform to imprint in the microchannel plate bottom surface. This impedes subsequent high resolution optical imaging through the glass. In addition, the glass sometimes sticks a little to the steel platform and also to the vacuum pumpout block, as these components become progressively oxidized with repeated use. This may induce glass cracking and does require frequent cleaning and resurfacing of the steel fixturing. For very long structures, (more than .about.40 cm) the glass on steel platform method is not viable, because of occasional cracking caused by large mismatches in thermal expansion and contraction. Also, the vacuum pumpout fixture of above-referenced application Ser. No. 09/039,522 was attached to the glass plate pair from the top on both ends, which made the loading and vacuum connection procedures awkward and fragile. This also imprints the top side on the ends with the steel finish texture.
The present invention provides several key improvements over the vacuum fusion bonding apparatus of above-referenced application Ser. No. 09/039,522, these including (1) the glass plates are oriented so that the outside surface of the microchannel plate only contacts oven atmosphere during the bonding cycle; (2) the addition of an interlayer plate, such as graphite, eliminates any sticking of the glass to the steel base platform or imprinting of the steel platform texture on the glass; (3) temporary clamps are used to hold the assembly together between alignment/assembly and transport/connection to the vacuum port in the oven; (4) the vacuum path is now downward from the microchannel plate to the oven vacuum line whereby the application of vacuum pulls the whole assembly together and downwards onto a stable platform, and (5) provides alignment and temporary clamping of the glass plates prior to bonding. Thus, the present invention provides an improved apparatus and method for vacuum fusion bonding of large area patterned glass plates, such as microstructure channel for chemical electrophoresis. The invention is for microchannel or other arbitrary patterned glass plates. Microfluidic channel is the application (e.g. electrophoresis, gas chromatography). This method may also apply to low pressure gas channels, with multiple electrode access and thus could enable certain fixed pattern electric discharge display systems, or ionizing photon/particle detection systems.